Users randomly kicked from groups

Strange issue that just popped up – no plugins updated (if it ain’t broke…). BP 1.5.3, S2Member, latest WP. A few users, after having requested membership to, and after having been approved for membership in a PRIVATE group, get randomly kicked out of the group after a day or so. I’m still troubleshooting, hoping that they just accidentally hit the ‘leave group’ button (I hope!). Their S2Member permissions continue to allow them to see the group so it’s not an S2Member issue – they just have to keep requesting membership and getting approved.

This seems to be happening to just a few members, and it seems to be happening in groups where I am the sole admin, and where there is another admin. All groups are private, all groups set to allow admins only to accept memberships or invite others.